Time for action – using watch variables and expressions

Finally, it's worth seeing what the Variables view can do.

  1. Create a breakpoint at the start of the execute() method.
  2. Click on the Hello World icon again.
  3. Highlight the openInformation() call and navigate to Run | Step Into Selection.
  4. Select the title in the the Variables view.
  5. Modify where it says Hello in the bottom half of the Variables view and change it to Goodbye:
    Time for action – using watch variables and expressions
  6. Save the value using Ctrl + S (or Cmd + S on OS X).
  7. Click on the Resume icon and the newly updated title can be seen in the dialog.
  8. Click on the Hello World icon again.
  9. With the debugger stopped in the execute() method, highlight event in the Variables view.
  10. Right-click on the value and choose Inspect (Ctrl + Shift + I or Cmd + Shift + I on an OS X) and the value is opened in the Expressions view:
    Time for action – using watch variables and expressions
  11. Click on the Add new expression option in the bottom of the Variables view.
  12. Add new java.util.Date() and the right-hand side will show the current time.
  13. Right-click on the new java.util.Date() and choose Re-evaluate Watch Expression. The right-hand pane shows the new value.
  14. Step through the code line by line, and notice that the watch expression is reevaluated after each step.
  15. Disable the watch expression by right-clicking on it and choosing Disable.
  16. Step through the code line by line and the watch expression will not be updated.

What just happened?

The Eclipse debugger has many powerful features, and the ability to inspect (and change) the state of the program is one of the more important ones.

Watch expressions, when combined with conditional breakpoints, can be used to find out when data becomes corrupted or used to show the state of a particular object's value.

Expressions can also be evaluated based on objects in the Variables view, and the code completion is available to select methods, with the result being shown with Display.

Pop quiz – debugging

Q1. How can an Eclipse plug-in be launched in debug mode?

Q2. How can certain packages be avoided when debugging?

Q3. What are the different types of breakpoints that can be set?

Q4. How can a loop debugged that only exhibits a bug after 256 iterations?

Q5. How can a breakpoint be set on a method when its argument is null?

Q6. What does inspecting an object do?

Q7. How can the value of an expression be calculated?

Have a go hero – working with breakpoints

Using the conditional breakpoint to stop at a certain method is fine if the data is simple, but sometimes there needs to be more than one expression. To implement additional functionality, the breakpoint can be delegated to a breakpoint() method in a Utility class. The following steps will help you to work with breakpoints:

  1. Create a Utility class with a static method breakpoint(), this will return a true value if the breakpoint should stop and false otherwise:
    public class Utility {
  public static boolean breakpoint() {
    System.out.println("Breakpoint");
    return false;
  }
}
  2. Create a conditional breakpoint in the execute() method, which calls Utility.breakpoint().
  3. Click on the Hello World icon again and the message will be printed to the host Eclipse's Console view. The breakpoint will not stop.
  4. Modify the breakpoint() method to return true instead of false. Run the action again. The debugger will stop.
  5. Modify the breakpoint() method to take the message as an argument, along with a Boolean value that is returned to say if the breakpoint should stop.
  6. Set up a conditional breakpoint with the following code:
    Utility.breakpoint(
 ((org.eclipse.swt.widgets.Event)event.trigger).stateMask
  != 0,"Breakpoint")
  7. Modify the breakpoint() method to take a varargs Object array, and use that in conjunction with the message to use String.format() for the resulting message:
    Utility.breakpoint(  ((org.eclipse.swt.widgets.Event)event.trigger).stateMask
  != 0,"Breakpoint" %s %h",event,
    new java.util.Date().getTime()))
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